(211e) Density Effects in Flow Properties Characterization of Powder Mixtures
Powders and granular materials can be found at any stage of processing in manufacturing in many industries, such as chemical, catalyst, food and pharmaceutical; they exhibit a variety of flow patterns, and their state and behavior differ from application to application. Since there is a lack of fundamental understanding of powder behavior, multiple problems can be encountered during production, such as jamming of the hoppers, sub-standard blending performance, and weight variability of final products due to segregation and/or agglomeration. Scale-up could also be a challenge, since the lack of constitutive equations for granular materials forces most scale-up efforts to follow the trial-and-error route. The field of powder characterization is employed as both, a distinguishing method for choosing the best-fit material and a predictive tool to analyze the process performance, and therefore, plays a very important role in process and product development. There are numerous methods to characterize the flow properties of granular materials, such as avalanching testers, fluidizers, shear cells, indicizers, density methods, angle of repose, etc.; however, most of them are application-specific, and it is not clear, how they correlate with each other or with process performance. For this reason, the use of most of these testers is restricted to a specific application, for which they were designed, and any attempts to apply the results of such experiments to a different application frequently result in process failure. In this respect, relating the variables and parameters outputted by various testers to the fundamental properties of materials is of great interest for successful implementation of powder characterization as a predictive tool for process optimal performance.
A method comparison study was undertaken to investigate the effects of water addition on flow and shear properties of a family of standard materials; this family of binary mixtures of materials was created to span a wide range of flow properties, from cohesive to free flowing. Various characterization methods were employed in this study, including an avalanching tester, called the Gravitational Displacement Rheometer (GDR), a rotational shear cell, compressibility tester and dynamic flow properties characterization suite of FT4 Powder Rheometer. The study determined that addition of water to studied materials affected the shear and flow properties of those materials in a very different manner, suggesting that water facilitated formation of particles structures in the blends.
In addition, a study was performed to investigate how densities obtained from these various characterization methods compare. It was found that the densities of the powders tested in most unconfined flow regime methods had a near-perfect linear correlation with one another, suggesting that there exists a critical ?dilated? density value that is independent of the measurement method and can be considered a material property. The densities of the powders in consolidated state correlated poorly, especially for the methods where consolidation mechanisms were not tightly controlled (i.e. tapping). Examination of densities as a function of applied normal stress in the shear cell revealed that this relationship tends to be more linear for the powders that are free flowing and more quadratic for the materials that are cohesive; suggesting a measurement of the entrapped air content in a powder to be of critical importance to flow properties evaluation.